Dynamic aspects of total body nitrogen metabolism are being explored in young adult humans, elderly people and obese subjects. This research program involves the application of 15N-labeled amino acids as tracers for studying whole body protein turnover and individual amino acid fluxes in human subjects. The model used to date has involved an analysis of enrichment of 15N in urinary urea following a 48-60 hour administration of 15N-glycine. The use of other labeled amino acids and models are being explored. The overall purpose of these studies is to increase an understanding of the nature of the adaptive responses of human protein metabolism in order to develop better measures of nutritional status and a more rational basis for assessing protein and energy requirements under conditions of health and in disease.